The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In the field of an automatic transmission, more multiple shift-stages are used to enhance fuel efficiency and drivability of a vehicle. Recently increased oil price leads to a more competition in enhancing fuel efficiency of a vehicle.
In this sense, research for the engine-downsizing has been conducted to reduce weight while enhancing fuel efficiency. Similarly, research on an automatic transmission has been performed to simultaneously provide better drivability and fuel efficiency through more shift stages with reduced the number of parts.
In order to achieve more shift stages for an automatic transmission, the number of parts, especially the number of planetary gear sets, is typically increased, which may deteriorate installability, production cost, weight and/or power flow efficiency.
An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to seven control elements (frictional elements), and thus may be easily lengthy transmission, thereby undermining installability.
We have discovered that disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements is sometimes attempted. However, such an arrangement may not be widely applicable, and using dog clutches may easily deteriorate shift-feel.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.